B54 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
hrown colour, particularly on the velum. A very 
delicate cuticle is also present. In the young Pecten 
the epidermal cells near the margin of the mantle 
and on its outer surface are very long compared with those 
of the epidermis elsewhere, and are evidently active 
secreting cells of the shell substance. In adult specimens 
this great difference is not seen. The columnar cells on 
the free margin of the mantle lobes, especially on the eye 
stalks (fig. 35), have a very peculiar appearance, due either 
to delicate connecting bridges like the * prickle cells” 
or to the walls having processes which interlock; most 
probably the former. Lying amongst these epithelial 
cells are numerous sense cells (‘‘ pinselzellen’’), to be 
described later in the chapter on the sense organs. 
Underlying the epidermis, there is at the margin of 
the mantle lobes (fig. +) a substantial connective tissue, 
consisting of delicate fibres with few scattered nuclei. 
There are numerous blood spaces in this layer, and the 
circumpalhal artery (fig. 4, A.c.) and the cireumpalhal 
nerve (fig. +, V.c.) pass through it, in close proximity, 
the blood vessel being situated on the shell side of the 
nerve. Passing inwards, away from the margins, the 
mantle lobes become extremely thin, the structure being. 
more and more trabeculated until, after passing the line 
of attachment of the pallial muscles, there is practically 
nothing between the epidermal layer of cells but bridges 
of fibrous tissue, large spaces being left in which are to 
be seen numerous blood corpuscles with large nuclei. 
Ramifying in the connective tissue before mentioned, 
at the margin of the mantle Jobes, are the pallial muscles 
(fies. abjierel ot VM ea DM.) | 
The pallial musculature of Pecten is both important 
and complex, and the edges of the mantle are very well 
supplied, owing to the energetic part played by the velum 
ee a a ee 
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